1. Field of the Invention
This invention generally relates to a differential gain and differential phase compensator circuit for a video signal. More particularly, the present invention relates to a differential gain and differential phase compensator circuit for a video signal suitable for use with a video tape recorder of a direct recording system employing more than two video heads more than two.
2. Description of the Prior Art
As the video tape recorder (VTR) of this kind, there is proposed such a VTR of two-phase recording system which employs a plurality of video heads, for example, four video heads to record a video signal on a tape in two-phase recording way. According to the two-phase recording system, the frequency of a record signal is lowered to a half (1/2.) Therefore, in a video signal S1 (see FIG. 1A), video signals S2 and S3 appearing within successive two 1H intervals (interval in one period of a horizontal synchronizing signal) are respectively expanded twice in timebase. And, the timebase-expanded video signals S2 and S3 (see FIGS. 1B and 1C) are alternately recorded on a tape in turn by two video heads (namely, A head and B head).
When the video signal is recorded on the tape at each one scanning line by alternately driving a plurality of video heads in turn, the characteristics of signal paths through the respective video heads are different from one another so that differences appear in a signal distortion such as gain change, phase change, frequency characteristic change and the like. As a result, when the recorded signal is reproduced, the characteristic regarding a reproduced image changes at each scanning line so that the picture quality of the reproduced image is inevitably deteriorated considerably on the whole.
Particularly when there is any change in the white level regarding the whole of the video signal, the brightness on the picture screen is also changed at each scanning line. Moreover, when the level of the chroma signal in the video signal is changed, the depth of color on the picture screen is also changed at every scanning line. Furthermore, when the phase of the chroma or color signal is changed, the hue on the picture screen is also changed at each scanning line. At any rate, a so-called line crawling phenomenon occurs on the picture screen and hence a stripe pattern appears on the picture screen.
Moreover, when the timebases of the signals reproduced by the A head and the B head are not exactly coincident with each other, a so-called hue change occurs at each scanning line so that color shades are caused to change.
Therefore, in order to solve the above problems, in the art there is proposed a method in which the whole of the video signal is successively superimposed with a pilot signal and then recorded to thereby give the pilot signal with a signal distortion which is exerted on the video signal upon recording. Upon reproducing, this pilot signal is extracted, the signal distortion which is effected on the video signal is detected by detecting various distortions exerted on the pilot signal and the reproduced video signal is compensated for on the basis of the detected results so that the signal distortion exerted upon expanding and recording can be removed.
However, according to the compensation method thus proposed, since the pilot signal is superimposed upon the whole of the video signal, as the frequency of the pilot signal, it is necessary to select the frequency outside the frequency region of the video signal. As a result, the frequency region of the record signal on the tape must be secured over a wide range. As compared with a case in which upon constructing the circuitry of the VTR each circuit is designed by only considering the improvement of the characteristic of the video signal, the above previously proposed method brings about the result that the video signal is deteriorated. And, in this case, since the tape speed must be raised, the tape consumption amount must also be increased.